This application discloses an invention that is related, generally and in various embodiments, to systems and methods for monitoring a process output with a highly abridged spectrophotometer.
There are various types of spectrophotometers, and some are significantly more expensive than others. The relatively high cost of such devices is generally related to the number of measurements the spectrophotometer is configured to make over a given range of light wavelengths. For example, some of the more expensive spectrophotometers are scanning spectrophotometers which are configured to scan a constant bypass function across a given spectrum, thereby measuring reflectances at a multitude of different points across the spectrum. With some scanning spectrophotometers configured to scan at intervals of less than one nanometer (e.g., 0.2 nanometers), such scanning spectrophotometers provide a wealth of spectral data that may be used to generate a spectral curve that accurately represents the sample being measured. As known to those skilled in the art, spectral curves may be used in a number of applications. For example, spectral curves may be utilized to control the quality and consistency of a process output, to determine the color of a process output, etc. Although the accuracy generally provided by such scanning spectrophotometers can be useful in various applications, their initial expense is cost-prohibitive for many applications.
In some applications, to reduce the initial expense, a high-quality abridged spectrophotometer can be used in lieu of a scanning spectrophotometer. In general, abridged spectrophotometers measure reflectance of an object at a fixed number of points over a range of wavelengths of light. The number of points varies based on the cost and quality of the abridged spectrophotometer. If an abridged spectrophotometer measures reflectance at a significant number of points across a measured range of wavelengths, it can often perform quite similarly to a scanning spectrophotometer. For example, an abridged spectrophotometer that provides 128 spectral bands across the visual spectrum (e.g., approximately 400 nm to 700 nm) would provide spectral curves that reasonably approximate the spectral curves generated from a scanning spectrophotometer on most reflective surfaces. However, as the number of spectral bands is reduced, the agreement between the spectral curve produced by a scanning spectrophotometer and an abridged spectrophotometer begins to diverge. Also, if the object being measured has sharp spectral transitions, as would be the case for the measurement of didymium glass, the abridged spectrophotometer may fail to measure spectral information that falls between two measured wavelengths, thereby compromising the accuracy of the resulting spectral curve.
As manufacturers try to develop more affordable spectrophotometers, they often reduce the number of spectral bands measured to reduce the cost of the spectrophotometer. For example, an industry standard has developed whereby many abridged spectrophotometers manufactured today measure only thirty-one spectral bands across the visual spectrum. Although the spectral curves provided by such abridged spectrophotometers are generally acceptable for a number of applications, the initial expense of such abridged spectrophotometers is still cost-prohibitive for many applications.
To address the need for a low-cost abridged spectrophotometer, some manufacturers have developed highly abridged spectrophotometers that provide less than thirty-one spectral bands across the visual spectrum. These low-cost abridged instruments also may have uneven spacing of the bandpass filters across the measured spectrum as lower cost components are employed. The agreement of the spectral curves produced by the highly abridged spectrophotometers and higher quality spectrophotometers is relatively poor on most measured materials. Therefore, the highly abridged spectrophotometers are not currently being utilized in applications which require the level of accuracy generally provided by the higher quality spectrophotometers.